1. Field of the Invention
The inventions disclosed and taught herein relate generally to earth boring drill bits, and more specifically are related to improved earth boring drill bits having a combination of fixed cutters and rolling cutters having cutting elements associated therewith, the arrangement of all of which exhibit improved drilling efficiency, as well as the operation of such bits.
2. Description of the Related Art
The present disclosure relates to systems and methods for excavating a earth formation, such as forming a well bore for the purpose of oil and gas recovery, to construct a tunnel, or to form other excavations in which the earth formation is cut, milled, pulverized, scraped, sheared, indented, and/or fractured, (hereinafter referred to collectively as “cutting”), as well as the apparatus used for such operations. The cutting process is a very interdependent process that typically integrates and considers many variables to ensure that a usable bore hole is constructed. As is commonly known in the art, many variables have an interactive and cumulative effect of increasing cutting costs. These variables may include formation hardness, abrasiveness, pore pressures, and elastic properties of the formation itself. In drilling wellbores, formation hardness and a corresponding degree of drilling difficulty may increase exponentially as a function of increasing depth of the wellbore. A high percentage of the costs to drill a well are derived from interdependent operations that are time sensitive, i.e., the longer it takes to penetrate the formation being drilled, the more it costs. One of the most important factors affecting the cost of drilling a wellbore is the rate at which the formation can be penetrated by the drill bit, which typically decreases with harder and tougher formation materials and wellbore depth into the formation.
There are generally two categories of modern drill bits that have evolved from over a hundred years of development and untold amounts of dollars spent on the research, testing and iterative development. These are the commonly known as the fixed cutter drill bit and the roller cone drill bit. Within these two primary categories, there are a wide variety of variations, with each variation designed to drill a formation having a general range of formation properties. These two categories of drill bits generally constitute the bulk of the drill bits employed to drill oil and gas wells around the world.
Each type of drill bit is commonly used where its drilling economics are superior to the other. Roller cone drill bits can drill the entire hardness spectrum of rock formations. Thus, roller cone drill bits are generally run when encountering harder rocks where long bit life and reasonable penetration rates are important factors on the drilling economics. Fixed cutter drill bits, including impregnated drill bits, are typically used to drill a wide variety of formations ranging from unconsolidated and weak rocks to medium hard rocks.
The roller cone bit replaced the fishtail bit in the early 1900's as a more durable tool to drill hard and abrasive formations (Hughes 1915) but its limitations in drilling shale and other plastically behaving rocks were well known. The underlying cause was a combination of chip-hold-down and/or bottom balling [Murray et al., 1955], which becomes progressively worse at greater depth as borehole pressure and mud weight increase. Balling reduces drilling efficiency of roller cone bits to a fraction of what is observed under atmospheric conditions [Pessier, R. C. and Fear, M. J., “Quantifying Common Drilling Problems with Mechanical Specific Energy and a Bit-Specific Coefficient of Sliding Friction”, SPE Conference Paper No. 24584-MS, 1992]. Other phenomena such as tracking and off-center running further aggravate the problem. Many innovations in roller cone bit design and hydraulics have addressed these issues but they have only marginally improved the performance [Wells and Pessier, 1993; Moffit, et al., 1992]. Fishtail or fixed-blade bits are much less affected by these problems since they act as mechanical scrapers, which continuously scour the borehole bottom. The first prototype of a hybrid bit [Scott, 1930], which simply combines a fishtail and roller cone bit, never succeeded commercially because the fishtail or fixed-blade part of the bit would prematurely wear and large wear flats reduced the penetration rate to even less than what was achievable with the roller cone bit alone. The concept of the hybrid bit was revived with the introduction of the much more wear-resistant, fixed-cutter PDC (polycrystalline diamond compact) bits in the 1980's and a wide variety of designs were proposed and patented [Schumacher, et al., 1984; Holster, et al., 1992; Tandberg, 1992; Baker, 1982]. Some were field tested but again with mixed results [Tandberg and Rodland, 1990], mainly due to structural deficiencies in the designs and the lack of durability of the first-generation PDC cutters. In the meantime, significant advances have been made in PDC cutter technology, and fixed-blade PDC bits have replaced roller cone bits in all but some applications for which the roller cone bits are uniquely suited. These are hard, abrasive and interbedded formations, complex directional drilling applications, and in general applications in which the torque requirements of a conventional PDC bit exceed the capabilities of a given drilling system. It is in these applications where the hybrid bit can substantially enhance the performance of a roller cone bit with a lower level of harmful dynamics compared to a conventional PDC bit.
In a hybrid type drill bit, the intermittent crushing of a roller cone bit is combined with continuous shearing and scraping of a fixed blade bit. The characteristic drilling mechanics of a hybrid bit can be best illustrated by direct comparison to a roller cone and fixed blade bit in laboratory tests under controlled, simulated downhole conditions [Ledgerwood, L. W., and Kelly, J. L., “High Pressure Facility Re-Creates Downhole Conditions in Testing of Full Size Drill Bits,” SPE paper No. 91-PET-1, presented at the ASME Energy-sources Technology Conference and Exhibition, New Orleans, Jan. 20-24, 1991]. The drilling mechanics of the different bit types and their performance are highly dependent on formation or rock type, structure and strength.
Early concepts of hybrid drill bits go back to the 1930s, but the development of a viable drilling tool has become feasible only with the recent advances in polycrystalline-diamond-compact (PDC) cutter technology. A hybrid bit can drill shale and other plastically behaving formations two to four times faster than a roller cone bit by being more aggressive and efficient. The penetration rate of a hybrid bit responds linearly to revolutions per minute (RPM) unlike that of roller-cone bits, which exhibit an exponential response with an exponent of less than unity. In other words, the hybrid bit will drill significantly faster than a comparable roller-cone bit in motor applications. Another benefit is the effect of the rolling cutters on the bit dynamics. Compared with conventional PDC bits, torsional oscillations are as much as 50% lower, and stick/slip is reduced at low RPM and whirl at high RPM. This gives the hybrid bit a wider operating window and greatly improves toolface control in directional drilling. The hybrid drill bit is a highly application-specific drill bit aimed at (1) traditional roller-cone applications that are rate-of-penetration (ROP) limited, (2) large-diameter PDC-bit and roller-cone-bit applications that are torque or weight-on-bit (WOB) limited, (3) highly interbedded formations where high torque fluctuations can cause premature failures and limit the mean operating torque, and (4) motor and/or directional applications where a higher ROP and better build rates and toolface control are desired. [Pessier, R. and Damschen, M., “Hybrid Bits Offer Distinct Advantages in Selected Roller-Cone and PDC-Bit Applications,” SPE Drilling & Completion, Vol. 26 (1), pp. 96-103 (March 2011)].
In the early stages of drill bit development, some earth-boring bits use a combination of one or more rolling cutters and one or more fixed blades. Some of these combination-type drill bits are referred to as hybrid bits. Previous designs of hybrid bits, such as described in U.S. Pat. No. 4,343,371, to Baker, III, have provided for the rolling cutters to do most of the formation cutting, especially in the center of the hole or bit. Other types of combination bits are known as “core bits,” such as U.S. Pat. No. 4,006,788, to Garner. Core bits typically have truncated rolling cutters that do not extend to the center of the bit and are designed to remove a core sample of formation by drilling down, but around, a solid cylinder of the formation to be removed from the borehole generally intact for purposes of formation analysis.
Another type of hybrid bit is described in U.S. Pat. No. 5,695,019, to Shamburger, Jr., wherein the rolling cutters extend almost entirely to the center. A rotary cone drill bit with two-stage cutting action is provided. The drill bit includes at least two truncated conical cutter assemblies rotatably coupled to support arms, where each cutter assembly is rotatable about a respective axis directed downwardly and inwardly. The truncated conical cutter assemblies are frusto-conical or conical frustums in shape, with a back face connected to a flat truncated face by conical sides. The truncated face may or may not be parallel with the back face of the cutter assembly. A plurality of primary cutting elements or inserts are arranged in a predetermined pattern on the flat truncated face of the truncated conical cutter assemblies. The teeth of the cutter assemblies are not meshed or engaged with one another and the plurality of cutting elements of each cutter assembly are spaced from cutting elements of other cutter assemblies. The primary cutting elements cut around a conical core rock formation in the center of the borehole, which acts to stabilize the cutter assemblies and urges them outward to cut a full-gage borehole. A plurality of secondary cutting elements or inserts are mounted in the downward surfaces of a dome area of the bit body. The secondary cutting elements reportedly cut down the free-standing core rock formation when the drill bit advances.
More recently, hybrid drill bits having both roller cones and fixed blades with improved cutting profiles and bit mechanics have been described, as well as methods for drilling with such bits. For example, U.S. Pat. No. 7,845,435 to Zahradnik, et al. describes a hybrid-type drill bit wherein the cutting elements on the fixed blades form a continuous cutting profile from the perimeter of the bit body to the axial center. The roller cone cutting elements overlap with the fixed cutting elements in the nose and shoulder sections of the cutting profile between the axial center and the perimeter. The roller cone cutting elements crush and pre- or partially fracture formation in the confined and highly stressed nose and shoulder sections.
While the success of the most recent hybrid-type drill bits has been shown in the field, select, specifically-design hybrid drill bit configurations suffer from lack of efficient cleaning of both the PDC cutters on the fixed blades and the cutting elements on the roller cones, leading to issues such as decreased drilling efficiency and balling issues in certain softer formations. This lack of cleaning efficiency in selected hybrid drill bits can be the result of overcrowded junk slot volume, which in turn results in limited available space for nozzle placement and orientation, the same nozzle in some instances being used to clean both the fixed blade cutters and the roller cone cutting elements, and inadequate space for cuttings evacuation during drill bit operation.
The inventions disclosed and taught herein are directed to drill bits having a bit body, wherein the bit body includes primary and secondary fixed cutter blades extending downward from the bit, bit legs extending downward from the bit body and terminating in roller cutter cones, wherein at least one of the fixed cutter blades is in alignment with a rolling cutter.